This invention relates to gaskets for a fuel cell, and more particularly to improved gasket compositions comprised of fluoric synthetic rubber which do not include the addition of metal oxide and/or metal hydroxide so as to prevent deterioration of the gaskets by phosphoric acid. The invention further pertains to an improved construction and sealing arrangement for a fuel cell, wherein a seal is comprised of a gasket and a pair of bonds which are constructed so as to minimize deterioration of the bonds.
It is well known that a fuel cell produces electrical energy from an reaction of an enriched or reformed fuel and oxygen. Hydrogen is typically used as the fuel and may be obtained by reforming a methanol-water mixture in a reformer that is comprised of a catalyst and a heater for vaporizing the unreformed fuel. Air is normally the source of oxygen for the fuel cell. Oftentimes, a plurality of cell units are stacked in series so as to increase the output voltage generating capacity of the fuel cell.
One type of cell unit is comprised of an anode, a cathode, and an electrolyte matrix impregnated with an electrolyte, such as phosphoric acid, interposed between the anode and cathode. Aqueous phosphoric acid is generally known to be an excellent electrolyte in that it is stable but has a low vapor pressure around 200.degree. C. and is also a good conductor. Phosphoric acid also rejects carbon dioxide, and at temperatures around 200.degree. C. the anode is able to withstand carbon containing impurities such as carbon monoxide in the fuel gas.
A gasket may be used to seal the outer edges of the anode, cathode and electrolyte matrix of each cell unit. Typically, this gasket is comprised of silicone rubber or fluoric synthetic rubber. Although these materials are able to be used at temperatures around 200.degree. C., previous compositions of such rubber materials tend to corrode in the presence of phosphoric acid at these temperatures. Previous compositions of fluoric synthetic rubber can also deteriorate at these high temperatures, which can decrease the elasticity of the rubber material.
The inventor has identified the source of this deterioration problem. The inventor has found that the cause of the deterioration is metal oxides and/or metal hydroxides, such as MgO, PbO or Ca(OH).sub.2, which are typically added to the fluoric synthetic rubber material with a cross linking agent for increasing the rate of formation of the molecular bridging structure of the rubber material. These metal oxides and/or metal hydroxides are sometimes present in the rubber material in the form of metallic salts and are also present in excess. Deterioration occurs when the metal oxides, metal hydroxides and/or metallic salts react with the phosphoric acid to form phosphates. Because of the strong affinity the phosphates have for the phosphoric acid, the rubber material absorbs a relatively large amount of phosphoric acid and swells. As a result, the fluoric synthetic rubber deteriorates and loses much of its elasticity.
It is therefore an object of this invention to provide improved gasket compositions for a fuel cell wherein the gasket materials will not corrode or deteriorate in the presence of phosphoric acid at temperatures around 200.degree. C.
The inventor has also discovered an improved composition for bonds which can be used in between the electrodes and the gasket to improve the seal around the outer edges of the cell unit. Typically, the bonds have poor chemical resistance and poor thermostability as a result of their low molecular weight and the presence of some reactive functional groups in their molecular structure. Moreover, the stress generated as a result of the difference in thermal expansion between the electrodes and the gasket at fuel cell operating temperatures around 200.degree. C., acts primarily on the bonds. As a result, the bonds tend to deteriorate relatively easily.
It is therefore another object of this invention to provide a sealing arrangement for a fuel cell, wherein one or more bonds are interposed between the electrodes and gasket of the fuel cell and are constructed so that any stress acting on the fuel cell will act primarily on the gasket so as to minimize deterioration of the bond.